1. Field of the Invention
This invention relates to a magnetic brush developing device for developing electrostatic latent images.
2. Description of the Prior Art
For example, in image formation apparatuses such as electrophotographic copying machines and the like, the known developing devices applicable to these copying machines have resorted to the powder-cloud method, the cascade method, the magnetic brush method or the like and these have their own distinct characteristics and have been put into use in various fields in accordance with those characteristics.
The powder-cloud method or the cascade method has a merit in reproducibility, i.e., line copy, of business documents due to the occurrence of the so-called edge effect whereby developing toner is concentrated on those portions of an electrostatic latent image on the latent image bearing surface of a photosensitive medium or the like which have a great gradient of electric field, namely, those portions of the latent image in which the density of the original image is discontinuous and such portions are emphatically reproduced. On the other hand, this presents itself as a demerit in the reproduction of ordinary tone images (images containing half-tone density), namely, in the reproduction of those portions of an original image in which density is continuously varied. Also, in both of the powder-cloud method and the cascade method, the necessity of widening the area of the latent image bearing surface which is acted on by developer has led to a disadvantage of bulkiness of the developing device itself.
In contrast, the magnetic brush method comprises causing a magnet to attract a developer composed of a mixture of iron powder carrier and developing toner, forming the developer in a brush-like shape in the magnetic pole portion, and causing such developer to frictionally contact an electrostatic latent image on the latent image bearing member, thereby developing the latent image to obtain a reproduction of an image. The iron powder itself acts as a soft developing electrode and thus, it enables the developing toner to be deposited in proportion to the charge density of the electrostatic latent image. Thus, the magnetic brush method is suited for the reproduction of toner images. Further, this method is featurized in that it permits compactness of the developing device itself.
As an improvement over the magnetic brush developing device, a system is known in which a non-magnetic member is disposed around a magnet bar having poles of the opposite polarities alternately arranged circumferentially thereof and these two are moved relative to each other to thereby form magnetic brushes in the surface portions of the non-magnetic member corresponding to the magnetic poles so that the magnetic brushes softly and frictionally touch the surface to be developed, thus developing a latent image thereon.
Such a magnetic brush developing device according to the prior art has involved a difficult problem in the reproducibility of the density of original image. The reason is that once the toner, which is deposited on a copying medium, usually a sheet of paper, has covered the copying medium in the form of one or two layers, the density of the copy image is no longer varied but assumes saturated condition even if the number of the toner layers is increased. Because of this, the copy image will be very hard in visual impression as compared with the original image. In order to soften the impression of such copy image in an effort to approximate it to that of the original image, the quantity of toner deposited must generally be decreased, whereas this has in turn resulted in reduced reproducibility of character images.
Also, in the device of this type, an effort to utilize the repelling magnetic fields of the magnetic means has often encountered difficulties in transporting the magnetic developing material from the developing container to the developing station and there is known no method which perfectly utilizes the repelling magnetic fields as well as the coaction thereof with the centrifugal force, gravity, inertia force, etc. of the developing material.
A solution to these problems is disclosed in U.S. Pat. No. 4,030,447 issued to the assignee in common with that of the present invention (see column 5, line 7 to column 8, line 21 and FIGS. 3(A) to 8), wherein there is shown a developing device having two magnetic poles of identical polarity disposed in opposed relationship with a latent image bearing surface. By so arranging the two poles of identical polarity, the developing toner transported with a non-magnetic cylinder (hereinafter referred to as the sleeve) is caused to form brushes at the pole positions to effect development peculiar to such brushes and between the poles, the developing toner jumps from the surface of the sleeve to the latent image surface by the repelling magnetic fields while it is downwardly dropped by the actions of centrifugal force, gravity and inertia force, as a result of which there may be achieved an effect peculiar to both the cascade development and the cloud development. However, if the latent image bearing member was rapidly moved to enhance the image formation speed, it has been found that the developing capability was reduced in the developing device using the above-described method.
On the other hand, a technique in which two magnetic brushes are opposed to a latent image bearing surface and one blanket of developing material is formed in order to achieve rapid development is known from U.S. Pat. No. 3,640,248. The specification (column 1, line 58 to column 3, line 75) and FIG. 2 of this patent show two non-magnetic sleeves disposed in spaced relation around a magnet having a pair of poles (N-S) so that one magnetic blanket of the developer is formed across the open space between the two sleeves.
Similarly, techniques in which two parallel sleeves are disposed in opposed relationship with an electrostatic latent image bearing surface are known from U.S. Pat. No. 3,543,720 (FIG. 1 and column 3, line 24 to column 4, line 43) and German Pat. No. 1218287 (FIG. 3 and column 5, line 11 to column 6, line 2).
The constructions of these three prior art publications are common with one another in that two substantially symmetric magnets and a sleeve surrounding them are disposed in entirely parallel, opposed relationship with the electrostatic latent image carrier, and particularly in that each of the magnets has a pair of N-S poles or a plurality of pairs of alternately arranged N-S poles. In other words, these constructions are intended simply to widen the developing zone and none of them shows a special contrivance of magnet pole arrangement which will provide qualitative differences between the configurations of the magnetic brushes on the respective sleeves and between the developing methods carried out thereby. For example, the aforementioned U.S. Pat. No. 3,543,720 teaches that the rotational velocity of the second sleeve is made slower than that of the first sleeve, but this is only intended to permit the more of toner to accumulate in the open space between the two sleeves and be brought into contact with the latent image bearing member, and in regard to the arrangement of the magnetic poles directly concerned with development and the polarities of these poles, it forms no difference from the methods of the conventional types in which different magnetic poles are alternately arranged.